Mixture of photoconductors in an active matrix

ABSTRACT

A photosensitive member having a binder layer comprising a mixture of photoconductive particles dispersed in an electrically active matrix material. Preferably, one of the photoconductive materials is cadmium sulfoselenide. The photoconductive particles comprise materials which exhibit the capability for photoexcited hole generation and injection, with the active organic matrix being substantially transparent and non-absorbing in the wavelength region of use and capable of transporting holes injected from the photoconductive particles. The member may be imaged in the conventional xerographic mode which includes charging, exposure to light, followed by development. The photosensitive layer has a coefficient of absorption between 1/L and 8/L wherein L is the thickness of the photoconductor in microns, the member is capable of continuous tone reproduction and has improved xerographic characteristics.

United States Patent Bean et al.

MIXTURE OF PHOTOCONDUCTORS IN AN ACTIVE MATRIX Inventors: Lloyd F. Bean, Rochester; Robert W. Gundlach, Victor, both of NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22 Filed: Apr. 12, I973 [21] Appl. No.: 350,666

Related US. Application Data [63] Continuation-in-part of Ser, No. 94,195, Dec. 1,

1970, abandoned.

[52] US. Cl 96/l.5; 252/501 [51] Int. Cl. G03q 5/06 [58] Field of Search 96/1 R, 1.5; 252/501 [56] References Cited UNITED STATES PATENTS 3,121,006 2/1964 Middleton et a1. 96/].5 3.121.007 2/1964 Middleton et a1. 6/15 3,281,240 10/1966 Cassiers et a1. i 96/1 5 3,542,545 11/1970 Goffe 96/15 X 3,598,582 8/1971 Herrick et a1 96/1.5

Primary Examiner-Roland E. Martin, Jr. Attorney, Agent, or Firm-James J. Ralabate; James P. OSullivan; Ronald L. Lyons [5 7] ABSTRACT A photosensitive member having a binder layer comprising a mixture of photoconductive particles dispersed in an electrically active matrix material. Preferably, one of the photoconductive materials is cadmium sulfoselenide. The photoconductive particles comprise materials which exhibit the capability for photoexcited hole generation and injection, with the active organic matrix being substantially transparent and non-absorbing in the wavelength region of use and capable of transporting holes injected from the photoconductive particles. The member may be imaged in the conventional xerographic mode which includes charging, exposure to light, followed by development. The photosensitive layer has a coefficient of absorption between l/L and 8/L wherein L is the thickness of the photoconductor in microns, the member is capable of continuous tone reproduction and has improved xerographic characteristics.

6 Claims, No Drawings MIXTURE OF PI-IOTOCONDUCTORS IN AN ACTIVE MATRIX BACKGROUND OF THE INVENTION This application is a continuation-in-part of copending application Ser. No. 94,195, filed Dec. 1, 1970, now abandoned.

This invention relates in general to xerography and more specifically to a novel photosensitive device and method of use.

In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely dividedelectroscopic marking particles on the surface of the photoconductive insulating layer.

A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated by U.S. Pat. No. 3,121 ,006 to Middleton and Reynolds which describes a number of binder layers comprising finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. In its present commercial form. the binder layer contains particles of zinc oxide uniformly dispersed in a resin binder and is coated on a paper backing.

In the particular examples of binder systems described in Middleton et al, the binder comprises a material which is incapable of transporting injected charge carriers generated by the photoconductor particles for any significant distance. As a result, with the particular materials disclosed in the Middleton et al patent, the photoconductor particles must be in substantially continuous particle-to-particle contact throughout the layer in order to permit the charge dissipation required for cyclic operation. With the uniform dispersion of photoconductor particles described in Middleton et al, therefore, a relatively high volume concentration of photoconductor, up to about 50 percent or more by volume, is usually necessary in order to obtain sufficientphotoconductor particle-to-particle contact for rapid discharge. It has been found, however, that high photoconductor loadings in the binder layers of the resin type result in the physical continuity of the resin being destroyed, thereby significantly reducing the mechanical properties of the binder layer. Layers with high photoconductor loadings are often characterized by a brittle binder layer having little or no flexibility.

.On the other hand, when the photoconductor concentration is reduced appreciably below about 50 percent by volume, the discharge rate is reduced, making high speed cyclic or repeated imaging difficult or impossible.

U.S. Pat. No. 3,121,007 to Middleton et al teaches another type of photoconductor which includes a two phase photoconductive binder layer comprising photoconductive insulating particles dispersed in a homogeneous photoconductive insulating matrix. The photoconductor is in the form of a particulate photoconductive inorganic crystalline pigment broadly disclosed as being present in an amount from about 5 to percent by weight. Photodischarge is said to be caused by the combination of charge carriers generated in the photoconductive insulating matrix material and charge carriers injected from the photoconductive crystalline pigment into the photoconductive insulating matrix.

U.S. Pat. No. 3,037,861 to Hoegl et a1 teaches that polyvinyl carbazole exhibits some long-wave U. V. sensitivity and suggests that its spectral sensitivity be extended into the visible spectrum by the addition of dye sensitizers. Hoegl et al further suggests that other additives such as zinc oxide or titanium dioxide may also be used in conjunction with polyvinyl carbazole. In Hoegl et al, it is clear tht the polyvinyl carbazole is intended to be used as a photoconductor, with or without additive materials which extend its spectral sensitivity.

In addition, certain specialized layer structures particularly designed for reflex imaging have been proposed. For example, U.S. Pat. No. 3,165,405 to Hoesterey utilizes a two layered zinc oxide binder structure for reflex imaging. The Hoesterey patent utilizes two separate contiguous photoconductive layers having different spectral sensitivities in order to carry out a particular reflex imaging sequence. The I-Ioesterey device utilizes the properties of multiple photoconductive layers in order to obtain the combined advantages of the separate photoresponse of the respective photoconductive layers.

It can be seen from a reveiw of the conventional composite photoconductive layers cited above, that upon exposure to light, photoconductivity in the layer structure is accomplished by charge transport through the bulk of the photoconductive layer, as in the case of vitreous selenium (and other homogeneous layer modifications). In devices employing photoconductive binder structures, which include inactive electrically insulating resins such as those described in the Middleton et a1, U.S. Pat. No 3,121,006, conductivity or charge transport is accomplished through high loadings of the photoconductive pigment allowing particle-to-particle contact of the photoconductive particles. In the case of photoconductive particles dispersed in a photoconductive matrix, such as illustrated by the Middleton et al, U.S. Pat. No. 3,121,007, photoconductivity occurs through the generation of charge carriers in both the photoconductive matrix and the photoconductor pigment particles.

Although the above patents rely upon distinct mechanism of discharge throughout the photoconductive layer, they generally suffer from common deficiencies in that the photoconductive surface during operation is exposed to the surrounding environment, and particularly in the case of cycling xerography, susceptible to abrasion, chemical attack, heat, and multiple exposures to light during cycling. These effects are characterized by a gradual deterioration in the electrical characteristics of the photoconductive layer resulting in the printing out of surface defects and scratches, localized areas of persistent conductivity which fail to retain an electrostatic charge, and high dark discharge.

In addition to the problems noted above, these photoconductive layers require that.the photoconductor compise either a hundred percent of layer. as in the case of the vitreous selenium layer, or that they preferably contain a high proportion of photoconductive material in the binder configuration. The requirements of a photoconductive layer containing all or a major proportion of a photoconductive material further restricts the physical characteristics of the final plate, drum or belt in that the physical characteristics such as flexibility and adhesion of the photoconductor to a supporting substrate are primarily dictated by the physical properties of the photoconductor, and not by the resin or matrix material which is preferably present in a minor amount.

Another form of composite photosensitive layer which has also been considered by the prior art ineludes a layer of photoconductive material which is covered with a relatively thick plastic layer and coated on a supporting substrate.

US. Pat. No. 3,041,166 to Bardeen describes such a configuration in which a transparent plastic material overlays a layer of vitreous selenium which is contained on a supporting substrate. The plastic material is described as one having a long range for charge carriers of the desired polarity. In operation, the free surface of the transparent plastic is electrostatically charged to give polarity. The device is then exposed to activating .radiation which generates a holeelectron pair in the photoconductive layer. The electronmoves through the plastic layer and neutralizes a positive charge on the free surface of the plastic layer thereby creating an electrostatic image. Bardeen, however, does not teach any specific plastic materials which will function in this manner, and confines his examples to structures which use a photoconductor material for the top layer.

French Pat. No. 1,577,855 to Herrick et a1 describes a special purpose composite photosensitive device adapted for reflex exposure by polarized light. One embodiment employs a layer of dichroic organic photoconductive particles arrayed in oriented fashion on a supporting substrate and a layer of polyvinyl carbazole formed over the oriented layer of dichroic material. When charged and exposed to light polarized perpendicularly to the orientation of the dichroic layer, the oriented dichroic layer and polyvinyl carbazole layer are both substantially transparent to the initial exposure light. When the polarized light hits the white background-of the document being copied, the light is depolarized, reflected back through the device and absorbed by the dichroic photoconductive material. In another embodiment, the dichroic photoconductor is dispersed in oriented fashion throughout the layer of polyvinyl carbazole.

It is also known that although prior art systems can provide excellent line copy and -'copies of other constrasty material having a relatively short dynamic range of about 0.6 density units when used in combination with conventional xerographic development processes such as cascade development, they at generally incapable of good continuous tone reproduction. The dynamic range of a particular xerographic plate and development system as used herein is intended to mean that range of original image densities which will produce a viewable change in the density of the reproduction produced by the plate where, the density D log l/R where R equals the ratio of reflected light to incident light. For example, in a very dense area of an original or reproduction where only one tenth of the incident light is reflected back to the eye of the viewer, R equals 1/10 and the log of l/R, i.e., density. of course, would be 1. A density of 1.3 is where about 1/20 of the incident light is reflected back to the viewer. Practically, reflection densities anywhere from about 1.2 to 1.5 or above appear to the human eye as a very dense black. Thus, generally it is thought that a given imaging system should have a dynamic range of about 1.2 or 1.5 or more to produce a quality tone reproduction, with a reasonably full latitude of contrast. It is known that conventional amorphous selenium plate xerography has serious limitations in this regard.

In view of the state of the art, it can readily be seen that there is a need for a general purpose photoreceptor exhibiting acceptable photoconductive characteristics such as total response and which additionally provides the capability of exhibiting outstanding physical strength and flexibility to be reused under rapid cyclic conditions without the progressive deterioration of the xerographic properties due to wear, chemical attack, and light fatigue.

OBJECTS OF THE INVENTION It is, therefore, an object of this invention to provide a novel photosensitive device which overcomes the above noted disadvantages.

It is a further object of this invention to provide a novel imaging system.

It is another object of this invention to provide a method of imaging the photosensitive member.

It is further object of this invention to provide a novel photosensitive binder structure.

It is another object of this invention to provide a novel binder structure characterized by an extremely low ratio of photoconductor to binder.

It is yet another object of this invention to provide a novel photosensitive member which is capable of exhibiting outstanding physical properties.

It is another object of this invention to provide a photosensitive layer which provides tonal control.

SUMMARY OF THE INVENTION Theforegoing objects and others are accomplished in accordance with this invention by providing a photosensitive member having a composite photosensitive layer which comprises unoriented photoconductive particles of at least two different materials dispersed in an electrically active organic binder or matrix. The photoconductive particles must be capable of generat' ing and injecting photo-excited holes into the electrically active organic material which comprises a transparent organic polymer or nonpolymeric material which is substantially non-absorbing to radiation in the spectral region of intended use, .but which is active in that it allows the injection of photo-excited holes from the photoconductive particles and allows these holes to be transported through the active matrix.

It should be understood that the active organic matrix material does not function as a photoconductor in the wavelength region of use. As stated above, holeelectron pairs are photogenerated in the photoconductive particles and the holes are then injected into the active matrix with hole transport occurring through the active matrix.

One embodiment of a typical application of the instant invention consists of a supporting substrate such as a conductor containing a binder layer thereon. For example, the binder layer may comprise particles of trigonal selenium and cadmium sulfoselenide contained in a transparent polymeric layer which is transparent and nonphotosensitive in the spectral ranges to which the selenium and cadmium sulfoselenide particles are responsive which allows for hole injection and transport. The transparent active (polymer) matrix allows one to take advantage of extremely low photoconductor loadings not previously available to the art and preferably certain selected matrix materials having high charge injection and transport efficiency are utilized. In addition, the structure can function effectively for forming continuous tone images. This structure can be imaged in the conventional photographic manner which usually includes charging. optical projection exposure, and development. A

As defined herein, a photoconductor is a material which is electrically photoresponsive -to light in the wavelength region in which it is to be used. More specifically, it is a material whose electrical conductivity increases significantly in response to the absorption of electromagnetic radiation in a wavelength region in which it is to be used. This definition is necessitated by the fact that a vast number of aromatic organic compounds are known or expected to be photoconductive when irradiated with strongly absorbed ultraviolet, xray, or gamma-radiation. Photoconductivity in organic materials is a common phenomenon. Practically all highly conjugated organic compounds exhibit some degree of photoconductivity under appropriate conditions. Most of these organic materials have their prime wavelength response in the ultraviolet. However, little commercial utility has been found for ultraviolet responsive materials, and their short wavelength response is not particularly suitable for document copying or color reproduction. In view of the general prevalence of photoconductivity, it is therefore necessary that for the instant invention, the term photoconductor or photocnductive be understood to include only those materials which are in fact substantially photoresponsive in the wavelength region in which they are to be used.

The active material, which is also referred to as the active matrix material when used as a matrix for the binder layer, is a substantially non-photoconductive material which transports at least about percent of the photoexcited holes generated at fields of about 2 X 10" volts/cm. This material is further characterized by the ability to transport the carrier at least 10 cm. at a field of no more than about IO volts/cm. In addition, the active matrix material is substantially transparent in the wavelength region in which the device is to be used.

As can be seen from the above discussion, most materials which are useful active matrices for binder layers of the instant invention are incidentally also photoconductive when radiation of wavelengths suitable for their electronic excitation, usually shorter than visible radiation is absorbed by them. However, photoresponse in the short wavelength region, which falls outside the spectral region for which the photoconductor is to be used, is irrelevant to the performance of the device. It is well known that radiation must be absorbed in order to excite photoconductive response, and the transparency criteria stated above for the active matrix materials implies that these materials do not contribute significantly to the photoresponse of the photoreceptor in the wavelength region of use.

The active transport material which is employed in the photoconductive layer in the instant invention is a material which is an insulator to the extent that an electrostatic charge placed on said active binder material is not conducted in the absence of illumination at a rate sufficient to prevent the formation and retention of an electrostatic latent image thereon. In general, this means that the specific resistivity of the active transport material should be at least 10 ohm-cms. The use of two phase systems in which the active material is substantially transparent to radiation in a region in which the photoconductor is to be used; for any absorption of desired radiation by the active material will prevent this radiation from reaching the photoconductive particlesor pigment where it is much more effectively utilized. It therefore follows that it is advantageous to use active matrix materials which are transparent in the wavelength in which the photoconductor or pigment has its'main response, and more particularly in the wavelength region in which the photoconductor is to be used. A detailed discussion of active matrix binders is contained in copending application Binder Imaging Member and Method, Ser. No. 93,994, filed Dec. 1, 1970, now abandoned, by M. Smith, C. F. l-lacket and R. W. Radler filed concurrently with this application, the disclosure of which is incorporated herein by reference.

Applications where complete transparency in the visible region is not required for the active material include the selective recording of narrow-band radiation such as that emitted from lasers, spectral pattern recognition, color coded form duplication, and possibly functional color xerography. Certain combinations of active matrix materials and various photoconductors can be selected to be of particular use for selective spectral response such as color separation without the use of optical filtering. The photosensitive layers of this invention may be self-suporting or may be coated on a substrate. Preferably, the substrate is a conductive material. Typical conductors comprise aluminum, steel, brass or the like. The substrate may be rigid or flexible and of any convenient thickness. Typical substrates include flexible belts or sleeves, sheets, webs, plates, cylinders, and drums. The substrate or support may also comprise a composite structure such as a relatively conductive plastic or paper base; plastic or paper coated with a thin conductive layer such as aluminum or copper iodide; or glass coated with a thin conductive coating of chromium or tin oxide. When using a transparent substrate it should be understood that imagewise exposure may optionally be carried out through the substrate or back of the imaging member.

The binder contains a mixture of particles of at least two photoconductive materials dispersed in an unoriented fashion in an electrically active matrix binder material. The photoconductive particles may consist of any suitable inorganic or organic photoconductors which are capable of injecting photoexcited holes into the matrix. Typical inorganic materials include inorganic crystalline compounds and inorganic photoconductive glasses. Typical inorganic crystalline compounds include cadmium sulfoselenide, cadmium selenide, cadmium sulfide, and mixtures thereof. Inorganic photoconductive glasses include amorphous selenium and selenium alloys such as selenium-tellurium and selenium arsenic. Selenium may also be used as a crystalline form known as trigonal selenium. Typical organic materials include phthalocyanine pigments such as the X-form of metal free phthalocyanine described in US. Pat. No. 3,357,989 to Bryne et al, metal phthalocyanines, such as copper phthalocyanine; quinacn'dones available from DuPont under the Tradename Monastral Red, Monastral Violet, and Monastral Red Y; substituted 2,4-diamino-triazines disclosed by Weinberger in US. Pat. No. 3,445,227; triphenodioxazine disclosed by Weinberger in US. Pat. No. 3,442,781; polynuclear aromatic quinones available from Allied Chemical Corp. under the Tradename Indofast Double Scarlet, Indofast Violet Lake B, Indofast Brilliant Scarlet, and lndofast Orange. The above list of photoconductors should in no way be taken as limiting, but is merely illustrative of suitable materials. The size of the photoconductive particles is not critical, but particles in a size range of about 0.01 to 1.0 microns yield particularly satisfactory results.

As previously stated, the photoconductive material of the instant invention is employed in an unoriented manner. By unoriented, it is meant that the pigment or photoconductive material is isotropic with respect to the exciting electromagnetic radiation, in that it is equally sensitive to exciting radiation of any polarization.

The active matrix material may comprise any suitable transparent organic polymer or nonpolymeric material capable of transporting photo-excited holes injected from the photoconductive pigment into it and allowing the transport of these holes through the active matrixto selectively] discharge a surface charge. Polymers having this characteristic have been found to contain repeating units of a polynuclear aromatic hydrocarbon which may also contain heteroatoms such as; for example, nitrogen, oxygen, or sulfur. Typical polymers include polyvinvyl carbazole (PVK), poly-l-vinylpyrene, (PVP, Polymethylene pyrene and N-substituted polymeric acrylic acid amides of pyrene. Typical nonpolymeric materials include carbazole, N-ethylcarbazole, N-phenylcarbazole, pyrene, tetraphene, lacetylpyrene, 2,3-benzochrysene, 6,7-benzopyrene, lbromopyrene, l-ethylpyrene, l-methylpyrene, perylene, 2-phenylindole, tetracene, picene, l,3,6,8-tetraphenylpyrene, chrysene, fluorene, fluorenone, phenanthrene, triphenylene, l,2,5,6-dibenzanthracene, l,2,3,4-dibenzanthracene, 2,3,-benzopyrene, 2,3,- benzochrysene, anthraquinone, dibenzothiophene, and naphthalene. In addition to the above, suitable mixtures of active polymer and/or active nonpolymeric materials may also be used.

It should be understood that the use of any polymer (a polymer being a large molecule built up by the repetition of small, simple chemical units) whose repeat unit contains the appropriate aromatic hydrocarbon, such as carbazole, and which supports hole injection and transport, may be used. It is therefore not the intent of the invention to restrict the type of polymer which can be employed as the matrix material. In addition, suitable mixtures of active polymers with inactive polymers or nonpolymeric materials may also be employed.

In general, the active layer is substantially transpar ent or non-absorbing in at least some significant portion of the range from about 4,()()8,0()() Ang., but will still function to allow injection and transport of holes generated within this wavelength range by the photoconductive pigment particles.

An upper limit on photoconductor volume concentration or occupancy is governed by various factors: Notably I the stage at which the physical properties of the polymer are seriously impaired; (2) the stage at which there is significant transport through particle-toparticle contacts; and (3) the stage at which, with conductive pigments such as trigonal selenium, there is excessive hole sweep out during charging. The latter two factors frequently lead to a lack of cycling ability. In general, to attain the best combination of physical and electrical properties, the upper limit for the photoconductive pigment or particles must be no greater than about 5 percent by volume of the binder layer. A lower limit for the photoconductive particles of about 0.5 percent by volume of the binder layer is required to insure that the light absorption coefficient is sufficient to give appreciable carrier generation. in order to achieve a closely equivalent discharge rate under both charging conditions, it is necessary that the average depth of photon absorption is near the center of the layer.

Layer thicknesses from about 2 to lOO microns have been found satisfactory, with a preferred thickness of about 5 to 50 yielding particularly good results.

Although the active material may comprise any suitable polymer or nonpolymeric material having the required properties, polymeric materials are preferred in that their physical properties such as flexibility, are superior to the physical properties of the nonpolymeric materials. A detailed discussion of the operation of photoconductive layers having active matrix binders is described in copending application entitled Binder Imaging Member and method by M. Smith, C. F. Hackett and R. W. Radler filed concurrently with the present application, the disclosure of which is incorporated herein by reference.

While it is not fully understood, it has been discovered that by dispersing a mixture of two or more finelydivided photoconductive pigments which can generate and inject holes into a binder which can transport holes in response to imagewise exposure an extended dynamic range photoconductor is provided which has a dynamic range which is dependent on both the absorption coefficient and the thickness of the photoreceptor. There are several mechanisms proposed which could explain the extension of the dynamic range. One possible mechanism is that two phenomena combine in a unique way. First the absorption coefiicient for the actinic radiation is increased substantially reducing the penetration of actinic light into the photoreceptor. At the same time, trapping sites are increased to the point that the mean distance traveled by a charge carrier is reduced to a small fraction of the total thickness of the photoconductor.

This combination of properties unexpectedly permits the various levels of depth in the photoconductive layer to operate independently, and yet respond to widely different values of exposure. For example, a point 15 ,u. below the surface of a photoconductor having an absorption coefficient, oz= 0.2 y. will receive only 5% of the light incident at the top surface. The absorption coefficient at is obtained from the following relationship: I J e" wherein 1,, is the original intensity and l is the intensity after passing through thickness t of a layer whose absorption coefficient is a. Only by limiting the carrier range, can discharge of the uppermost levels take place without interfering with the independent dis charge of the lower layers taking place at a considerably reduced rate.

Another possible mechanism involves the use of photosensitive particles whose photogeneration and injection properties are strongly field dependent. Here it is speculated that the structure provides a relatively slow carrier transport as distinguished from carrier trapping during imaging exposure resulting in an internal field distribution which again has a lesse internal field near the surface of the photoconductor than at points further away from the surface. This field distribution coupled with the fact that the photogeneration and photoinjection properties of the particles are strongly field dependent provides a rapid reduction in effective photosensitivity as discharge progresses, giving rise to an extended dynamic range.

Either theory explains why, short of particle-toparticle contact, increased pigment ratios result in increased dynamic range. Also, predictably, increased photoconductor thickness has been found to increase the dynamic range of this new type of photoreceptor.

The absorption coefficient for the photoconductive members of this invention are at least UL and not more than 8/L where L is the photoconductor thickness in microns with an optimum being about 2/L to 4/L the preferred value being about US. That is, for a 15 micron plate, a coefficient of 3/15 or about 0.2 p. is desired.

The photoconductive members of this invention also have a mean charge carrier travel of from about 1/10 to US the photoconductor thickness, preferably about 1/5.

It was found that generally mixtures of pigments in photoconductive binders produced a xerographic plate which showed a residual sensitivity pattern or sensitivity fatigure corresponding to recent previous exposures of the plate which resulted in a ghost image corresponding to prior exposures. in order to eliminate the sensitivity fatigue problem, it was necessary to dark rest the plate for several hours or to heat the plate to a temperature of 50C for several minutes. It has been found surprisingly, that CdSSe dispersed in a active matrix organic binder, in any suitable weight ratio, preferably from about one-half parts to about 5 parts by weight to 100 parts binder produces a xerogrpahic photoconductor layer which is simultaneously resistive highly photosensitive, recyclible, ambipolar, exhibits low fatigue, is relatively panchromatic and exhibits a long dynamic range providing continuous tone imaging. It was found that the addition of CdSSe in the preferred range given above to active matrix binders containing low concen trations of phthalocyanine particles substantially reduced residual conductivity or photoconductivity patterns which were found to increase with cycling when CdSSe was not present and rendered the plate ambipolar as compared to photoconductive insulating layers without CdSSe. Without the CdSSe the plates are not recyclible to both polarities of charge because of ghost images.

The pigments may be incorporated in active matrix binders typically by incorporating them in a dissolved or melted binder material by any suitable means such as ball milling. These methods also include strong shear agitation, preferably with simultaneous grinding, roller milling, sand milling, ultrasonic agitation, high speed blending and any desirable combination of these methods.

The pigment-binder solvent dispersion slurry (or the pigment-binder-melt) may be applied to conductive substrates by any of the well known painting or coating methods including spray coating, flow coating, knife coating, electro coating, Mayer bar draw down, dip coating, reversal coating and so on. Spraying in an electric field may be preferred for smoothest finish and dip coating for convenience in the laboratory. The setting, drying and/or curing steps for these plates are generally similar to those recommended for films of the particular binders used for the other painting applications.

The preferred composition for the layer comprises from about one-half to about 5 parts by weight cadmiumsulfoselenide, from about one-half to about 5 parts by weight phthalocyanine and from about 1 to about 5 parts by weight selenium dispersed in about parts of active matrix binder of polycarbazole. The total of the CdSSe, phthalocyanine and selenium should not exceed about 15 parts by weight based on 100 parts of photoconductive binder to provide all of the advantages of this invention.

It has been found that when the concentration of the CdSSe is above about one-half parts by weight, or below about 10 parts by weight the plate is ambipolar, has good recyclible properties, and ghost images are reduced. By ambipolar is meant that the plate may be recycled when charged to either polarity.

When the layer contains more than about one-half part by weight phthalocyanine, the plate has increased sensitivity to red light. When the layer contains up to about 3 parts by weight of phthalocyanine, the plate is ambipolar, is recyclable showing little or no charge fatigue and is able to produce a high quality continuoustone image.

When the layer contains more than about 1 percent by weight of selenium, the plate has increased sensitivity to blue light; and is ambipolar. When up to about 8 parts by weight selenium is present in the layer, the plate is recyclible showing little or no charge fatigue and is ambipolar.

The plates of this invention have a brightness acceptance range which is considerably greater than selenium. Tests have shown the brightness acceptance range of the photoconductors of this invention to also be greater than that for selenium alloys.

The photoconductive plates of this invention are eminently more useful for producing continuous tone images than selenium. lt has also been found that the brightness acceptance range of the plates of this invention vary with the thickness of the plates. This is not true of selenium and its alloys providing a further control over the results obtainable with the plates of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following Examples further specifically define the surprisingly advantageous photoconductive insulating material of this invention. The parts and percentages are by weight unless otherwise indicated.

The Examples below are intended to illustrate various preferred embodiments of the improved photoconductive insulating material of this invention.

The photoconductive members of Examples l2l shown in Table l of this invention are made as follows:

A 1/] mix of cyclohexanone and toluene is prepared. About 100 parts by weight of polyvinylcarbazole is dissolved in about 560 parts by weight of the cyclohexanone-toluene mix. The dissolving rate may be increased by heating. The solution is then placed in a milling jar and the desired amount of CdSSe, phthalocyanine and seleniumare added. The materials are milled by rotating the jar until the particles of CdSSe, phthalocyanine and selenium have a particle size of less than about one micron. The slurry is coated onto a clean Xerox 914 Copier aluminum drum to produce a thickness of 15 microns when dried. Drying is accomplished using forced air at 75C in about an hour. The drum is then placed in a Xerox 914 Copier for testing.

The phthalocyanine is preferably metal-free and in either beta or X polymorphic forms prepared as shown in US. Pat. No. 3,357,989 to Byrne et al. The cadmiumsulfoselenide is available as 1020 Red Pigment from General Color Co., Fort Wayne, Indiana.

In the Examples shown in Table I, the parts given are parts by weight based on 100 parts by weight polyvinylcarbazole.

or more in their light sensitivity depending on whether they are charged negatively or positively.

Column 7 is an evaluation again rated from 1 to 10 of the quality of the images formed in regard to whether a ghost of previous image is formed which is evidence of residual conductivity or photoconductivity. The plates rated from 8 to 10 have no visible ghost even when exposed to intense radiation. A rating of from 5 to 7 indicates that ghosting can be eliminated if the plate is rested and/or heated. A rating of four or less indicates that a ghost persists indefinately which results from attack on the plate.

Column 8 is an evaluation based on a rating of from 1-10 of the ability of the photoconductor to make high quality continuous tone images. The rating is arbitrary with the best plates having a rating of from 8 to 10. A

TABLE I 1 2 3 4 5 6 7 8 Example Parts Parts Parts Ambi- Ghost Continuous No. CdSSe Sc Phthalo Recyclibility polarity lmagcs Tonc l O 5 1 2 4 1 7 2 V2 5 l 2 4 2 7 3 l 5 l 3 6 6 7 4 2 5 l 6 7 8 8 5 3 5 l 8 8 l l0 6 5 l 7 8 l() 9 7 7 5 l 2 6 l 6 8 3 O l 6 8 10 8 9 3 l l 6 6 l0 8 1O 3 2 /2 1 8 l0 l0 9 l l 3 l0 1 4 6 l() 4 l2 3 l 2 2 l0 3 l3 3 5 O 6 8 1O 5 l4 3 5 Va 4 8 l() 7 l5 3 5 2 /2 2 10 1O 7 l6 3 5 5 4 4 8 4 17 3 5 1O 1 2 4 1 l8 3 5 5 4 4 8 4 l9 3 0 O 7 5 9 8 2O 0 5 6 7 8 7 2l 0 O l 3 4 3 8 In the Table I, the first column is the Experiment 40 rating of from 5-7 indicates marginal images. A rating number. The second-fourth columns are the parts of cadmiumsulfoselenide, selenium and phthalocyanine (X-form) based on 100 parts of polyvinylcarbazole in the photoconductive layer. Column 5, Recyclibility, represents the qualities of the photoconductor which make it useful as a reusable photoconductor and represents fatigue, dark decay, charge acceptance and residual potential. The photoconductors are given a rating of from 1 to 10 with 10 representing the plates having the best recyclible characteristics. For example, a plate having a rating of less than 5 although suitable for manual use is not useful in a reusable plate machine in that the plates require a rest period and/or heating between cycles to restore photoconductive properties. A plate having a rating of from 5 to 7 requires about three minutes between cycles but can be used at a faster rate if a uniform negative charge is applied to the plate between the cycles. A rating of from 8 to 10 indicates that the plates are good enough to be used in a rapid continuous imaging machine environment such as the Xerox 720, 914 and 2400 copiers.

Column 6 is an evaluation of the ambipolarity characteristic again being given a rating of from 1 to 10 with the best plates showing no appreciable difference in recyclibility, ghosting or quality of continuous tone images when charged to either polarity. A rating of less tha 5 indicates that the plates vary by a factor of four of from 1-4 indicates unacceptable continuous tone imaging.

Although specific components and proportions have been stated in the above description of the preferred embodiments of the invention, other suitable materials as listed herein may be used with similar results. In addition, other materials may be added to materials used in order to synergize or enhance or otherwise modify the properties of the novel photoconductive layer of this invention. For example, if desired, the spectral response of the novel photoconductive layers of this invention may be further modified by including photosensitizing dyes.

Also, after formation of the electrostatic latent image on the photosensitive layers of this invention, the image may be utilized in a number of ways. One typical utilization mode is rendering the image visible by the xerographic development techniques of contacting the latent image areas with a finely divided marking material called toner that is brought into surface contact with the surface of the plate and is held there electrostatically in a pattern corresponding to the electrostatic latent image. Cascade development, for example, as disclosed in Walkup et al US. Pat. No. 2,638,416 as well as any other suitable mode of bringing toner into contact with the electrostatic latent image known to those skilled in the art of xerography may be used herein.

Another mode of utilizing the electrostatic latent images formed on the imaging members hereof is to transfer the charge pattern to another layer by bringing the two' layers into very close proximity and utilizingbreakdown techniques as described, for example, in Carlson U.S. Pat. No. 2,982,647 and Walkup U.S. Pat. Nos. 2,825,814 and 2,937,943. For example, the layer to which the charge image is transferred may be a surface deformable material which may be caused to deform in image configuration as disclosed in Gunther et al U.S Pat. No. 3,l96,0l l.

The electrostatic latent image may also be directly read out utilizing devices such as electrometers which detect potential differences which may be translated into giving the graphic information that was represented by the original electrostatic latent image.

Insulating receiving sheets may be brought into contact with the electrostatic latent image bearing plates hereof and the receiving sheet developed with toner utilizing techniques which permit a plurality of such copies to be made from one master electrostatic latent image.

As disclosed in copending application Ser, No. 867,049, filed Oct. 16, 1969 relatively more conductive image receiving sheets including paper may be placed in contact with the electrostatic latent image bearing plates thereof inducing an image in said receiving sheet which induced image can be developed by techniques which permit 100 or more such developed receiving sheets to be mmade from a single master electrostatic latent image.

It will be understood that various other changes in the details, materials and steps which have been herein described and illustrated in order to explain the nature of this invention will occur to and may be made by those skilled in the art upon a reading of this disclosure and such changes are intended to be included within the prinicple and scope of this invention.

What is claimed is:

l. A photoconductive layer comprising an unoriented mixture of finely divided photoconductive particles comprising from about one-half to about parts by weight cadmium sulfoselenide and from about l to about 5 parts by weight selenium dispersed in 100 parts by weight of an active binder matrix material, said photoconductive layer having a thickness of about 2 to 100 microns, with the photoconductive particles being present in a total combined concentration of about 0.5 to 5 percent by volume of the photoconductive layer, the mixture being capable of generating holes in response to imagewise radiation, the binder being capable of transporting holes injected from the photoconductive particles during imagewise exposure, with the binder material being selected from the group consisting of;

polyvinyl carbazole; polyl-vinylpyrene; polymethylene pyrene; an N-substituted polymeric acrylic acid amide of pyrene; carbazole; N-ethylcarbzole;

N-phenylcarbazole; pyrene; tetraphene; lacetylpyrene; 2,3-benzochrysene; 6,7- benzopyrene; l-bromopyrene; l-ethylpyrene; l-

methylpyrene; perylene; Z-phenylindole; tetracene; picene; l,3,6,8-tetraphenylpyrene; chrysene; fluo' rene; fluorenone; phenanthrene; triphenylene; l,2,5 ,o-dibezanthracene; l,2,3 .4-

dibenzanthracene; 2,3-benzopyrene; 2,3- benzochrysene; anthraquinone; dibenzothiophene; naphthalene; and mixtures thereof, with said photoconductive layer exhibiting an absorption coefficient of at least l/L and not more than 8/L where L is the photoconductor layer thickness in microns. 2. A photoconductive layer comprising an unoriented mixture of finely divided photoconductive particles comprising from about one-half to about 5 parts of cadmium sulfoselenide and from about one-half to about 5 parts by weight phthalocyanine dispersed in parts by weight of an active binder matrix material, said photoconductive layer having a thickness of about 2 to 100 microns, with the photoconductive particles being present "in a total combined concentration of about 0.5 to 5 percent by volume of the photoconductive layer, the mixture being capable of generating holes in response to imagewise radiation, the binder being capable of transporting holes injected from the photoconductive particles during imagewise exposure, with the binder material being selected from the group consisting of;

polyvinyl carbazole; poly-l-vinylpyrene; polymethylene pyrene; an N-substituted polymeric acrylic acid amide of pyrene; carbazole; N-ethylcarbazole; N-phenylcarbazole; pyrene; tetraphene; lacetylpyrene; 2,3-benzochrysene, 6,7- benzopyrene, l-bromopyrene, l-ethylpyrene; lmethylpyrene; perylene; Z-phenylindole; tetracene; picene; l,3,6,8-tetraphenylpyrene; chrysene; fluorene; fluorenone; phenanthrene; triphenylene; l ,2,5 ,6-dibenzanthracene; l,2,3,4- dibenzanthracene; 2,3 -benzopyrene; 2 ,3

benzochrysene; anthraquinone; dibenzothiophene; naphthalene; and mixtures thereof, with said photoconductive layer exhibiting an absorption coefficient of at least l/L and not more than 8/L where L is the photoconductive layer thickness in microns.

3. A photoconductive layer comprising an unoriented mixture of finely divided photoconductive particles comprising from about one-half to about 5 parts cadmium sulfoselenide about one-half to 5 parts phthalocyanine, and from about 1 to about 5 parts by weight selenium dispersed in 100 parts by weight of an active binder matrix material, said photoconductive layer having a thickness of about 2 to 100 microns, with the photoconductive particles being present in a total combined concentration of about 0.5 to 5 percent by volume of the photoconductive layer, the mixture being capable of generating holes in response to imagewise radiation, the binder being capable of transporting holes injected from the photoconductive particles during imagewise exposure with the binder material being selected from the group consisting of;

polyvinyl carbazole; poly-l-vinylpyrene; polymethylene pyrene; an N-substituted polymeric acrylic acid amide of pyrene; carbazole; N- phenylcarbazole; pyrene; tetraphene; l acetylpyrene; 2,3-benzochrysene; 6,7-

benzopyrene; l-bromopyrene; l-ethylpyrene; lmethylpyrene; perylene; 2-phenylindole; tetracene; picene; l,3,6,8-tetraphenylpyrene; chrysene; fluorene; fluorenone; phenanthrene; triphenylene; l ,2,5,6-dibenzanthracene; 1,2,13,4- dibenzanthracene; 2,3-benzopyrene; 2,3-

benzochrysene; anthraquinone; dibenzothiophene;

naphthalene; and mixtures thereof, with said photoconductive layer exhibiting an absorption coefficient of at least UL and not more than 8/L where L is the photoconductor layer thickness in microns. 4. A photoconductive layer comprising an unoriented mixture of finely divided photoconductive particles comprising from about one-half to about 5 parts cadmium sulfoselenide, about one-half to 5 parts phthalocyanine, and from about 1 to about 5 parts by weight selenium dispersed in 100 parts by weight of an active binder matrix material, said photoconductive layer having a thickness of about 2 to 100 microns, with the photoconductive particles being present in a total combined concentration of about 0.5 to 5 percent by volume of the photoconductive layer, said mixture being capable of generating holes in response to imagewise radiation, the binder being capable of transporting holes injected from the photoconductive particles during imagewise exposure, with the binder material being selected from the group consisting of;

polyvinyl carbazole; poly-l-vinylpyrene, polymethylene pyrene; and N-substituted polymeric acrylic acid amide of pyrene; carbazole; N- phenylcarbazole; pyrene; tetraphene; l acetylpyrene; 2 ,3-benzochrysene', 6,7-

benzopyrene; l-bromopyrene;' l-ethylpyrene; lmethypyrene; perylene; 2-phenylindole; tetracene; picene; l,3,6,8-tetraphenylpyrene; chrysene; fluorene; fluorenone; phenanthrene; triphenylene', l ,2,5,6-dibenzanthracene; 1 ,2,3,4- dibenzanthracene; 2,3-benzopyrene; 2,3-

tive particles comprising from about one-half to about 5 parts cadmium sulfoselenide about onehalf to 5 parts phthalocyanine, and from about 1 to about 5 parts-by weight selenium dispersed in parts by weight of an active binder matrix material, said photoconductive layer having a thickness of about 2 to 100 microns, with the photoconductive particles being present in a total combined concentration of about 0.5 to 5 percent by volume of the photoconductive layer, the mixture being capable of generating holes in response to imagewise radiation, the binder being capable of transporting holes injected from the photoconductive particles during imagewise exposure with the binder material being selected from the group consisting of polyvinyl carbazole, poly-l-vinylpyrene; polymethylene pyrene, an N-substituted polymeric acrylic acid amide of pyrene; carbazole; N- phenylcarbazole; pyrene; tetraphene; l acetylpyrene; 2,3-benzochrysene; 6,7-

benzopyrene; l-bromopyrene; l-ethylpyrene; lmethylpyrene; perylene; 2-phenylindole; tetracene; picene; 1,3,6,8-tetraphenylpyrene; chrysene; fluorene; fluorenone; phenanthrene; triphenylene; l ,2,5,6-dibenzanthracene; 1 ,2,3,4- dibenzanthracene; 2,3-benzopyrene; 2 ,3-

benzochrysene; anthraquinone, dibenzothiophene; naphthalene; and mixtures thereof, with said photoconductive layer exhibiting an absorption coefficient of at least UL and not more than 8/L where L is the photoconductor layer thickness in microns;

b. uniformly electrostatically charging said layer;

0. exposing said charged layer to a pattern of radiation to which the particles are sensitive and to which the active matrix binder is substantially nonabsorbing until an electrostatic image is formed.

6. The method of claim 5 wherein said electrostatic image is developed to form a visible image.

Page 1 of 2 UNITED STATES PATENT @FFKQE QERTEFDCATE @l @QRREQ'MUN PATENT NO. I 3 899, 329

DATED August 12, 1.975

INVENTOR(S) Lloyd Fr, Bean, Robert W, Gundlach It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown beiow:

Column 2 line 161 delete "tht" and insert that.

Column 3 lines 22 & 231 delete "charged to give" and insert --charged to a given- Column 3, line 24, delete "holeelectron" and insert -=--hole--electron--rw Column 3 lines 51 & 52, delete "constrasty and insert --contrasty-o Column 3, line 55, delete "er" and insert are Column 4, line 28, insert --a between is and further.

Column 5 line 36, delete "photocnductive" and insert photoconductive-.,

Column 6 lines 62 & 631 delete "selenium arsenic" and insert selenium--arsenic-.

Column 7, line 281 delete selectivelyl" and insert selectively -o Column 7 line 34, delete (PVP and insert --(PVP) Column 7 line 34, delete "Polymethylene and insert polymethylene -o Column 8, line 29, delete method" and insert Method- Column 8 lire 60, delete "lml e' n ins rt -i l e' UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO.

DATED |NV,ENTOR(S) [SEAL] It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, interferring..

Column line 5, delete "lesse" and insert lesser.

Column line 32, delete "fatigure" and insert fatigue.

Column lines 9 & l0, delete "cadmiumsulfoselenide and insert cadmium sulfoselenide.

Column 10, line 28, delete "recyclable" and insert recyclible-.

Column lines ll & l2, delete "cadmiumsulfoselenide" and insert cadmium sulfoselenide--.

Column line 42 delete cadmiumsulfoselenide" and insert cadmium sulfoselenide-.

Column last line, delete "the" and insert -than--.

Column line 32, delete "made" and insert made.

Column line 60, delete "N-ethylcarbzole" and insert Nethylcarbazole.

Column 13 last line delete "1,2, 5,6-dibezanthracene" and insert -l,2, 5,6-dibenzanthracene.

Column 15, line 27, delete "methypyrene" and insert methylpyrene.

August 12,

Lloyd F. Bean,

Page 2 of 2 Robert W. Gundlach line 64, delete "interfering and insert Column 16, line 2, insert a after sulfoselenide.

Arrest:

RUTH C. MASON Arresting Officer C. IARSIIALLDANN Commission" oflalents and Trademarks 

1. A PHOTOCONDUCTIVE LAYER COMPRISING AN UNORIENTED MIXTURE OF FINELY DIVIDED PHOTOCONDUCTIVE PARTICLES COMPRISING FROM ABOUT ONE-HALF TO ABOUT 5 PARTS BY WEIGHT CADMIUM SULFOSELENIDE AND FROM ABOUT 1 TO ABOUT 5 PARTS BY WEIGHT SELENIUM DISPERSED IN 100 PARTS BY WEIGHT OF AN ACTIVE BINDER MATRIX MATERIAL, SAID PHOTOCONDUCTIVE LAYER HAVING A THICKNESS OF ABOUT 2 TO 100 MICRONS, WITH THE PHOTOCONDUCTIVE PARTICLES BEING PRESENT IN A TOTAL COMBINED CONCENTRATION OF ABOUT 0.5 TO 5 PERCENT BY VOLUME OF THE PHOTOCONDUCTIVE LAYER, THE MIXTURE BEING CAPABLE OF GENERATING HOLES IN RESPONSE TO IMAGEWISE RADIATION, THE BINDER BEING CAPABLE OF TRANSPORTING HOLES INJECTED FROM THE PHOTOCONDUCTIVE PARTICLES DURING IMAGEWISE EXPOSURE, WITH THE BINDER MATERIAL BEING SELECTED FROM THE GROUP CONSISTING OF, POLYVINYL CARBAZOLE, POLY-1-VINYLPYRENE, POLYMETHYLENE PYRENE, AN N-SUBSTITUTED POLYMERIC ACRYLIC ACID AMIDE OF PYRENE, CARBAZOLE, N-ETHYLCARBZOLE, N-PHENYLCARBAZOLE, PYRENE, TERAPHENE, 1-ACETYPYRENE, 2,3-BENZOCHRYSENE, 6,7-BENZOPYRENE, 1-BROMOPYRENE, 1-ETHYLPYRENE, 1METHYLPYRENE, PERYLENE, 2-PHENYLINDOLE, TETRACENE, PICENE, 1,3,6,8-TETRAPHENYLPYRENE, CHRYSENE, FLUORENE, FLUORENONE, PHENANTHRENE, TRIPHENYLENE, 1,2,5,6-DIBEZANTHRACENE, 1,2,3,4-DIBENZANTHRACENE, 2,3-BENZOPYRENEE, 2,3-BENZOCHRYSENE, ANTHRAQUINONE, DIBENZOTHIOPHENE, NAPHTHALENE, AND MIXTURES THEREOF, WITH SAID PHOTOCONDUCTIVE LAYER EXHIBITING AN ABSORPTION COEFFICIENT OF AT LEAST 1/L AND NOT MORE THAN 8/L WHERE L IS THE PHOTOCONDUCTOR LAYER THICKNESS IN MICRONS.
 2. A photoconductive layer comprising an unoriented mixture of finely divided photoconductive particles comprising from about one-half to about 5 parts of cadmium sulfoselenide and from about one-half to about 5 parts by weight phthalocyanine dispersed in 100 parts by weight of an active binder matrix material, said photoconductive layer having a thickness of about 2 to 100 microns, with the photoconductive particles being present in a total combined concentration of about 0.5 to 5 percent by volume of the photoconductive layer, the mixture being capable of generating holes in response to imagewise radiation, the binder being capable of transporting holes injected from the photoconductive particles during imagewise exposure, with the binder material being selected from the group consisting of; polyvinyl carbazole; poly-1-vinylpyrene; polymethylene pyrene; an N-substituted polymeric acrylic acid amide of pyrene; carbazole; N-ethylcarbazole; N-phenylcarbazole; pyrene; tetraphene; 1-acetylpyrene; 2,3-benzochrysene, 6,7-benzopyrene, 1-bromopyrene, 1-ethylpyrene; 1-methylpyrene; perylene; 2-phenylindole; tetracene; picene; 1,3,6,8-tetraphenylpyrene; chrysene; fluorene; fluorenone; phenanthrene; triphenylene; 1, 2,5,6-dibenzanthracene; 1,2,3,4-dibenzanthracene; 2,3-benzopyrene; 2,3-benzochrysene; anthraquinone; dibenzothiophene; naphthalene; and mixtures thereof, wiTh said photoconductive layer exhibiting an absorption coefficient of at least 1/L and not more than 8/L where L is the photoconductive layer thickness in microns.
 3. A photoconductive layer comprising an unoriented mixture of finely divided photoconductive particles comprising from about one-half to about 5 parts cadmium sulfoselenide about one-half to 5 parts phthalocyanine, and from about 1 to about 5 parts by weight selenium dispersed in 100 parts by weight of an active binder matrix material, said photoconductive layer having a thickness of about 2 to 100 microns, with the photoconductive particles being present in a total combined concentration of about 0.5 to 5 percent by volume of the photoconductive layer, the mixture being capable of generating holes in response to imagewise radiation, the binder being capable of transporting holes injected from the photoconductive particles during imagewise exposure with the binder material being selected from the group consisting of; polyvinyl carbazole; poly-1-vinylpyrene; polymethylene pyrene; an N-substituted polymeric acrylic acid amide of pyrene; carbazole; N-phenylcarbazole; pyrene; tetraphene; 1-acetylpyrene; 2,3-benzochrysene; 6,7-benzopyrene; 1-bromopyrene; 1-ethylpyrene; 1-methylpyrene; perylene; 2-phenylindole; tetracene; picene; 1,3,6,8-tetraphenylpyrene; chrysene; fluorene; fluorenone; phenanthrene; triphenylene; 1, 2,5,6-dibenzanthracene; 1,2,3,4-dibenzanthracene; 2,3-benzopyrene; 2,3-benzochrysene; anthraquinone; dibenzothiophene; naphthalene; and mixtures thereof, with said photoconductive layer exhibiting an absorption coefficient of at least 1/L and not more than 8/L where L is the photoconductor layer thickness in microns.
 4. A photoconductive layer comprising an unoriented mixture of finely divided photoconductive particles comprising from about one-half to about 5 parts cadmium sulfoselenide, about one-half to 5 parts phthalocyanine, and from about 1 to about 5 parts by weight selenium dispersed in 100 parts by weight of an active binder matrix material, said photoconductive layer having a thickness of about 2 to 100 microns, with the photoconductive particles being present in a total combined concentration of about 0.5 to 5 percent by volume of the photoconductive layer, said mixture being capable of generating holes in response to imagewise radiation, the binder being capable of transporting holes injected from the photoconductive particles during imagewise exposure, with the binder material being selected from the group consisting of; polyvinyl carbazole; poly-1-vinylpyrene, polymethylene pyrene; and N-substituted polymeric acrylic acid amide of pyrene; carbazole; N-phenylcarbazole; pyrene; tetraphene; 1-acetylpyrene; 2,3-benzochrysene; 6,7-benzopyrene; 1-bromopyrene; 1-ethylpyrene; 1-methypyrene; perylene; 2-phenylindole; tetracene; picene; 1,3,6,8-tetraphenylpyrene; chrysene; fluorene; fluorenone; phenanthrene; triphenylene; 1, 2,5,6-dibenzanthracene; 1,2,3,4-dibenzanthracene; 2,3-benzopyrene; 2,3-benzochrysene; anthraquinone, dibenzothiophene; naphthalene; and mixtures thereof, with said photoconductive layer exhibiting an absorption coefficient of at least 1/L and not more than 8/L where L is the photoconductive layer thickness in microns.
 5. A method of imaging which comprises: a. providing a photoconductive layer comprising an unoriented mixture of finely divided photoconductive particles comprising from about one-half to about 5 parts cadmium sulfoselenide about one-half to 5 parts phthalocyanine, and from about 1 to about 5 parts by weight selenium dispersed in 100 parts by weight of an active bindeR matrix material, said photoconductive layer having a thickness of about 2 to 100 microns, with the photoconductive particles being present in a total combined concentration of about 0.5 to 5 percent by volume of the photoconductive layer, the mixture being capable of generating holes in response to imagewise radiation, the binder being capable of transporting holes injected from the photoconductive particles during imagewise exposure with the binder material being selected from the group consisting of polyvinyl carbazole, poly-1-vinylpyrene; polymethylene pyrene, an N-substituted polymeric acrylic acid amide of pyrene; carbazole; N-phenylcarbazole; pyrene; tetraphene; 1-acetylpyrene; 2,3-benzochrysene; 6,7-benzopyrene; 1-bromopyrene; 1-ethylpyrene; 1-methylpyrene; perylene; 2-phenylindole; tetracene; picene; 1,3,6,8-tetraphenylpyrene; chrysene; fluorene; fluorenone; phenanthrene; triphenylene; 1, 2,5,6-dibenzanthracene; 1,2,3,4-dibenzanthracene; 2,3-benzopyrene; 2,3-benzochrysene; anthraquinone, dibenzothiophene; naphthalene; and mixtures thereof, with said photoconductive layer exhibiting an absorption coefficient of at least 1/L and not more than 8/L where L is the photoconductor layer thickness in microns; b. uniformly electrostatically charging said layer; c. exposing said charged layer to a pattern of radiation to which the particles are sensitive and to which the active matrix binder is substantially non-absorbing until an electrostatic image is formed.
 6. The method of claim 5 wherein said electrostatic image is developed to form a visible image. 